1. Field of the Invention
This invention relates to a servo adjustment method and an apparatus for servomotor which drives machine tools or industrial robots, and more particularly to a servo adjustment method and an apparatus which improve a delay in response to an instructed position for reversing of the servomotor to improve accuracy of position control.
2. Prior Art
In NC machine tools, a control system for a servomotor which drives moving body such as table, etc., is provided with a position control loop which makes the deviation between a NC position instruction signal and a feedback signal provided by a position transducer which detects the position of the moving body reduced to zero. In the servo control system, a velocity control loop obtains a velocity feedback signal by differentiating position feedback signal provided by the position control loop, and sends an output of speed loop gain as a torque signal to the servomotor.
For the purpose of carrying out circular arc cutting by the machine tool, two servomotors which drive a feed axis respectively are simultaneously controlled to allow the moving body for a circular interpolation feed motion. When rotating direction of the motor is reversed, moving direction of the moving body cannot be responsively reversed. For this reason, a delay in a response to the instructed position takes place. When the movable body moves along a circular path from one quadrant to an adjacent quadrant, there takes place a response error such that an actual path of the movable body deviates and bulges out from a desired path specified by a path command. Such phenomenon is called stick motion or quadrant protrusion and is one of the main cause of the deterioration of accuracy in machining.
The reason why such a phenomenon takes place is presumed as follows. It is necessary to invert torque signal to match with frictional torque when the motion direction is reversed. However, a feed shaft drive mechanism is temporarily stopped due to a delay caused by the response characteristic of the velocity loop.
A conventional compensation method which has been employed in reducing the response error produced at the moment of the reversal of the servomotor, includes steps as follows. An instructed torque value at the time of the reversal of the servomotor is detected, to determine a reference value of a compensation torque for compensation on the basis of the detected torque signal by trail and error. An operator carries out parameter-input procedures for inputting values of compensation torque into the NC unit.
However, according the above method, the operator determines the values of compensation torque in dependency upon his own skills and experiences. As a result, there were variations (unevenness) in result of the compensation in dependency upon the degree of skillfulness of the operator.
In addition, in determining compensation torque, value of suitable compensation torque was empirically determined while measuring error in response delay by using an instrument for measuring the accuracy of the circular interpolation feed motion. For this reason, high level skillfulness was required for that work.
In addition, since errors caused by response delay is varied according to the condition such as feed speed or circular arc radius, etc., even when the same machine is employed, parameter-inputting procedure for compensating torques had to be repeated for every condition. As a result, that work was troublesome.